A fractal-based correlation for time-dependent surface diffusivity in porous adsorbents

Fluid-solid adsorption processes are mostly governed by the adsorbate transport in the solid phase and surface diffusion is often the limiting step of the overall process in microporous materials such as zeolites. This work starts from a concise review of concepts and models for surface transport and variable surface diffusivity. It emerges that the phenomenon of hindered surface diffusion for monolayer adsorption, which is common in zeolites, and models able to fit a non-monotonic trend of surface diffusivity against adsorbate solid phase concentration, have received limited attention. This work contributes to the literature of hindered diffusion by formulating a time-dependent equation for surface diffusivity based on fractal dynamics concepts. The proposed equation takes into account the contributions of both fractal-like diffusion (a time-decreasing term) and hopping diffusion (a time-increasing term). The equation is discussed and numerically analyzed to testify its ability to reproduce the possible different patterns of surface diffusivity vs. time

Particle-wall interactions in entrained-flow slagging gasifiers

This paper aimed at the development of a phenomenological model of the fate of coal/ash particles in entrained-flow slagging coal gasifiers, which considers the establishment of a particle segregated phase in the near-wall region of the gasifier. In particular, near-wall phenomena were investigated and mechanistic understanding of particle–wall interaction patterns in entrained-flow gasifiers was pursued using the tool of physical modeling. Montan wax was used to mimic, at atmospheric conditions, particle-wall interactions relevant in entrained-flow gasifiers. As a matter of fact, this wax had rheological/mechanical properties resembling under molten state, those of a typical coal slag and, under solid state, those of char particles. Experiments have been carried out in a lab-scale cold entrained-flow reactor, equipped with a nozzle whence molten wax atomized into a mainstream of air to simulate the near-wall fate of char/ash particles in a real hot environment. The four particle-wall interaction regimes were investigated. The partitioning of the wax droplets/particles into the different phases was characterized by their selective collection at the reactor exhaust. Results showed that the particlewall interaction mechanisms and segregation patterns are deeply affected by the stickiness of both the wall layer and the impinging particle. In particular, the micromechanical interaction of a particle with a sticky wall enhances particle transport to the wall and the tendency to reach a segregationcoverage regime with the formation of a dense-dispersed phase in the near-wall region of the reactor. Furthermore, increasing the mainstream air flow rate induces particle segregation and accumulation phenomena

Sulfur Uptake by Limestone-Based Sorbent Particles in CFBC: The Influence of Attrition/Fragmentation on Sorbent Inventory and Particle Size Distribution

This paper presents a population balance model aiming at the prediction of sorbent inventory and particle size distribution establishing at steady state in the bed of an air-blown CFBC fuelled with a sulphur-bearing solid fuel. The core of the model is represented by population balance equations on sorbent particles which embody terms expressing the extent/rate of sorbent attrition/fragmentation. The effect of the progress of sulphation on attrition and fragmentation is taken into account by selection of appropriate constitutive equations. Model results are presented and discussed with the aim of clarifying the influence of particle attrition/fragmentation on sorbent inventory and particle size distribution, partitioning of sorbent between fly and bottom ash, sulphur capture efficiency. A sensitivity analysis is carried out with reference to relevant operational parameters of the combustor

TG, FT-IR and NMR characterization of n-C16H34 contaminated alumina and silica after mechanochemical treatment

This paper deals with the application of mechanochemistry to model systems composed of alumina or silica artificially contaminated with n-C16H34. The mechanochemical treatment was carried out by means of a ring mill for times ranging from 10 to 40 h. Thermogravimetry and infrared and nuclear magnetic resonance spectroscopies were used for the characterization of the mechanochemical products. The results have indicated that, in the case of alumina, almost all the contaminant n-C16H34 undergoes a complex oxidative reaction path whose end products are strongly held on the surface. These end products are most likely made of crosslinked, partially oxidized hydrocarbon chains bond to the solid surface via COO− groups. In the case of silica, the hydrocarbon undergoes a different, equally complex reaction path, but to a lower extent. In this case the end products are most probably carbonylic compounds and graphitic carbon. Then, for both solid matrices, the mechanochemical treatment promotes significant modification of the chemical nature of the polluting hydrocarbon with end products much more difficult to remove from the surface. As the systems studied are models of sites contaminated by aliphatic hydrocarbon, the results are worthy of consideration in relation to the mobility of the contaminants in the environment

THE INFLUENCE OF PARTICLE ATTRITION ON SORBENT INVENTORY AND PARTICLE SIZE DISTRIBUTION IN AIR-BLOWN CIRCULATING FLUIDIZED BED COMBUSTORS

This paper presents a population balance model aiming at the prediction of sorbent inventory and particle size distribution establishing at steady state in the bed of an air-blown circulating fluidized bed combustor fuelled with a sulphur-bearing solid fuel. The core of the model is represented by population balance equations on sorbent particles which embody terms expressing the extent/rate of sorbent attrition/fragmentation. The effect of the progress of sulphation on attrition and fragmentation is taken into account by selection of appropriate constitutive equations. Model results are presented and discussed with the aim of clarifying the influence of particle attrition/fragmentation on sorbent inventory and particle size distribution, partitioning of sorbent between fly and bottom ash, sulphur capture efficiency. A sensitivity analysis is carried out with reference to relevant operational parameters of the combustor

GRANULAR FLOW SIMULATIONS OF LIMITING REGIMES OF PARTICLES–WALL INTERACTION RELEVANT TO SLAGGING COAL GASIFIERS

In pilot entrained-flow slagging coal gasifiers, high conversion efficiency and low pollutant emission levels have been observed, but the mechanism leading to this behaviour is not fully understood. Recent literature proposes several different mechanisms as playing an important role, ranging from the sticking properties of both particles and slag-covered walls to the thermal and chemical history along the trajectory of the particles in the entire gasifier. Nonetheless, very few attention has been devoted to the role of particle–particle interactions, even if it has been shown that this mechanism can lead to new regimes likely to occur in slagging gasifiers and to promote the rise in the coal conversion efficiency. This study presents the results of a simplified configuration that allows to highlight the role of the four different interactions that can be envisaged when considering particles and confining walls as either sticky or non sticky. Particles are subjected to a body force that mimics the action of the drag exerted by a swirling flow field in a cylindrical vessel. Particle–particle collisions are modelled with an Hertzian approach that includes torque and cohesion effects. Results clearly indicate the different structure of the layer of particles establishing on the wall surface in the different interaction regimes. They confirm the importance to adequately take into account particle–particle interactions for a correct prevision of the fate of coal particles in slagging gasifiers

Simulating particle-wall micromechanical interaction in entrained-flow slagging gasifiers by cold impact experiments

This paper deals with particle–wall interaction phenomena in entrained-flow slagging coal gasifiers. Different micromechanical char–slag interaction patterns may establish, depending on the stickiness of the wall layer and of the impinging char particle. Micromechanical interaction patterns were studied by means of an appropriate experimental apparatus which permitted to record a single particle/droplet impact on a flat surface. Montan wax was used to simulate, at nearly ambient temperature, the char–slag rebound characteristics upon colliding the wall. Particle–wall collision was described in terms of the particle restitution coefficient. In particular, the influence of the particle temperature and the impact angle as well as the impact velocity and the target surface on the rebound characteristics was studied. Results highlighted that the particle restitution coefficient decreased when enhancing operating conditions (temperature and impact velocity) able to promote plastic deformation upon particle impact

Effect of steam on the performance of Ca-based sorbents in calcium looping processes

Calcium looping, a post-combustion “carbon capture and storage” process (see Figure 1), is usually carried out by means of a limestone-based sorbent in a dual interconnected fluidized bed reactor. The two stages of this process are limestone calcination and carbonation: in the former case, water vapor can be present as a product of the auxiliary fuel combustion needed to drive this endothermal step; in the latter case, water vapor is usually present in the combustion flue gas stream bearing the CO2 to be captured. This work pursues previous research concerning the hydration-induced reactivation of spent sorbents (1,2,3) further and aims at investigating the effect of the presence of water vapor on the performance of a limestone-based sorbent, with particular reference to the attrition/fragmentation tendency. To this end, experimental tests were carried out in a lab-scale apparatus, under typical operating conditions in terms of temperature and gas composition. The role of water vapor in changing the sorbent CO2 capture capacity (with respect to a base-case operation in which water vapor was absent) and the attrition/fragmentation tendency was examined (see, for example, Figure 2 up and down, respectively). Results from CO2 capture will be complemented with characterization of sorbent particles, by means of scanning electron microscopy, porosimetric and X-ray diffraction analyses. Please click Additional Files below to see the full abstract